The present invention relates to a filter apparatus and filter element for use with the filter apparatus, and more particularly to a filter apparatus preferably provided as an HEPA (High Efficiency Particulate Air), ULPA (Ultra-Low Penetration Air) or super ULPA filter apparatus and a filter element therefor.
Along with recent advances in scientific technologies and changes of life style, clean spaces and purified air have been increasingly required. It is natural that purified air is desired in hospitals and homes, and various air cleaners are used thereof. This also is true in precision instrument and food industries. Furthermore, in fabrication of integrated circuits and semiconductors, pharmaceuticals and manufacture of such medical products as artificial organs, an allowable amount of dust is much less than that in usual clean spaces, and generally an HEPA filter, preferably a ULPA filter and more preferably a filter apparatus classified as a super ULPA filter are required.
Such a filter apparatus used for air cleaning is adapted to have a filter element consisting of a filter material through which air is passed so that such air is cleaned and dust is removed therefrom.
An example of such filter element is illustrated in perspective view in FIG. 18. Such filter element 1 consists of a filter material 3 which is a filter medium bent or folded to form plural folds or ridges 2, for example, a filter cloth of fiberglass. Additionally, spacers 4 generally are placed between the ridges in order to uniformly position the filter material (only two spacers are illustrated). A filter apparatus is formed by air-tightly fitting the circumference of the filter element 1 in a rectangular frame (not shown). The air passing through the filter apparatus flows in a direction from the right rear, as shown in FIG. 18, and passes through the filter material to the left front. Such a filter element is described, for instance, in pp. 40 to 41 of "Development of high-function filter" (Osaka Chemical Research Series, Vol. 5, No. 9, Osaka Chemical Marketing Center).
A concept of filtration area is one standard for evaluating the performance of a filter apparatus. More specifically, the performance of a filter apparatus is measured by filtration area per unit volume of a filter element. Generally, for obtaining higher performance with less pressure loss, a filter element having the largest possible filtration area with the smallest possible size is preferred.
In the filter element 1 of FIG. 18, the total area of filter material 3 corresponds to the filtration area. In order to increase the filtration area of a filter element in such structure to improve the performance of the filter apparatus, a spacing between the ridges or folds, that is a pitch (length p in FIG. 18), is generally reduced to a minimum so that the ridges are formed in wrinkle-like shape.
However, in many cases reduction of the pitch p is disadvantageous. Thus, reduction of the pitch p is limited by the flexibility of the material itself, which depends on types of filter materials used. Also, adjacent filter materials (or a filter material and separator when a separator is provided) may be in contact with each other when the pitch is reduced excessively, whereby air passages needlessly are narrowed, the result being an increase of pressure loss.
For example, when an unwoven fiberglass cloth conventionally used for air filters is employed as a filter material for a filter element of the structure shown in FIG. 18, such unwoven cloth being 0.5 mm in thickness, the limit of the pitch is considered to be about 5 mm. Therefore, with a filter element 610 mm.times.610 mm in frontage (lengths a.times.b in FIG. 18) and 150 mm in depth (length c of FIG. 18), the filtration area is approximately 16 m.sup.2.
In addition, when fiberglass is used as a filter material, fine dust is caused from the fiberglass (Japanese Unexamined Patent Publication No. 3-34967). Thus, it is not an optimum material for obtaining clean air.